Problem: What is $20\%$ of $400$ ?
Explanation: Having $20\%$ of something means that you get $20$ out of every $100$ We can set up a proportion to find out what number is $20\%$ of $400$ $ \dfrac{{\text{percent}}}{100} = \dfrac{{\text{part}}}{{\text{whole}}}$ Which things do we know, and what are we trying to find? We know the ${\text{percent}}$ is $20$ . Is $400$ the ${\text{part}}$ or the ${\text{whole}}$ The $400$ is the ${\text{whole}}$ . We are trying to find the ${\text{part}}$ that makes up $20\%$ of it: $ \dfrac{{20}}{100} = \dfrac{{\text{part}}}{{400}}$ If we multiply the denominator of the fraction on the left by $4$ , it will be the same denominator of the fraction on the right. To keep things equal, let's also multiply the numerator on the left by $4$ $ \dfrac{{20} \times 4}{100 \times 4} = \dfrac{{\text{part}}}{{400}}$ $ \dfrac{{80}}{400} = \dfrac{{\text{part}}}{{400}}$ $ {80} = {\text{part}}$ So $80$ is $20\%$ of $400$.